DNV-RP-F111: Interference Between Trawl Gear and...

RECOMMENDED PRACTICE

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DNV-RP-F111

INTERFERENCE BETWEEN TRAWL GEAR AND PIPELINES

OCTOBER 2010

FOREWORDDET NORSKE VERITAS (DNV) is an autonomous and independent foundation with the objectives of safeguarding life,property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification andconsultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carriesout research in relation to these functions.

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The Standards and Recommended Practices are offered within the following areas:A) Qualification, Quality and Safety MethodologyB) Materials TechnologyC) StructuresD) SystemsE) Special FacilitiesF) Pipelines and RisersG) Asset OperationH) Marine OperationsJ) Cleaner EnergyO) Subsea Systems

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Recommended Practice DNV-RP-F111, October 2010Changes –

ACKNOWLEDGEMENTSThe following companies sponsored this JIP:

— Hydro ASA— Statoil— DNV.

The following organisations have made major contributions tothe hearing process:

— Reinertsen Engineering— Subsea 7.

DNV is grateful for the valuable input, including test results,cooperation and discussions with these companies.

CHANGES

• GeneralAs of October 2010 all DNV service documents are primarilypublished electronically.In order to ensure a practical transition from the “print” schemeto the “electronic” scheme, all documents having incorporatedamendments and corrections more recent than the date of thelatest printed issue, have been given the date October 2010.An overview of DNV service documents, their update statusand historical “amendments and corrections” may be foundthrough http://www.dnv.com/resources/rules_standards/.• Main changesSince the previous edition (October 2006), this document hasbeen amended, most recently in April 2009. All changes havebeen incorporated and a new date (October 2010) has beengiven as explained under “General”.

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Recommended Practice DNV-RP-F111, October 2010 – Changes

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Recommended Practice DNV-RP-F111, October 2010 Contents –

CONTENTS

1. GENERAL .............................................................. 71.1 Introduction .............................................................71.2 Trawling aspects ......................................................71.3 Scope and application .............................................81.4 Limitations ...............................................................91.5 Relationship to other rules......................................91.6 References ................................................................91.7 Trawl-pipeline interaction phases..........................91.8 Units ........................................................................101.9 Symbols and abbreviations...................................11

2. TRAWL DESIGN DATA .................................... 122.1 Types of trawl gear................................................122.2 Basic data requirements .......................................122.3 Most critical trawl gear.........................................132.4 Trawl gear impact frequency ...............................13

3. IMPACT................................................................ 133.1 Introduction ...........................................................133.2 Impact energy ........................................................133.3 Energy dissipation .................................................133.4 Simplified response calculations ..........................15

4. PULL-OVER ........................................................ 174.1 Introduction ...........................................................174.2 Structural modelling .............................................174.3 Pull-over loads for trawl boards and

beam trawls ............................................................174.4 Pull-over loads for clump weights........................184.5 Pull-over duration Trawl boards and

beam trawls ............................................................19

4.6 Pull-over duration - clump weight....................... 194.7 Response calculations .......................................... 20

5. HOOKING............................................................ 205.1 Introduction........................................................... 205.2 Structural model ................................................... 215.3 Snagging................................................................. 215.4 Critical span height ............................................... 215.5 Part penetration .................................................... 215.6 Wedged................................................................... 215.7 Maximum warp line force .................................... 215.8 Response calculations ........................................... 21

6. ACCEPTANCE CRITERIA............................... 226.1 General ................................................................... 226.2 Pipe shell - impact ................................................. 226.3 Coating - impact .................................................... 226.4 Pull-over................................................................. 236.5 Hooking .................................................................. 24

7. COATING IMPACT TESTING......................... 247.1 General ................................................................... 247.2 Specimens............................................................... 247.3 Test equipment ...................................................... 257.4 Test procedure....................................................... 267.5 Test-rig calibration ............................................... 26

8. BIBLIOGRAPHY ................................................ 27

APP. A ADVANCED IMPACT CALCULATION METHOD ........................................................... 28

APP. B EXAMPLE.......................................................... 31

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Recommended Practice DNV-RP-F111, October 2010 – Contents

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Recommended Practice DNV-RP-F111, October 2010

1. General1.1 IntroductionFishing activities such as bottom trawling shall be consideredfor offshore pipelines for two main reasons:

— possible hazard and inconvenience to the fishermen incase of trawl gear hooking to the pipeline, and

— possible hazard to the integrity of the pipeline due to loadsfrom the trawl gear.

This Recommended Practice (RP) covers the aspects of pipe-line integrity and not the potential hazard for fishermen in par-ticular.Equipment used for bottom trawling can expose a pipeline tosubstantial loads that may damage it. Such load is associatedwith the instantaneous impact and the subsequent pull-over asthe trawl gear hits and is dragged over the pipeline. In addition,hooking of trawl equipment may impose considerable loadingto the pipeline.Typical trawl gears are illustrated in Figures 1-1 to 1-3.

Figure 1-1Typical otter trawl gear crossing a pipeline

Figure 1-1 shows a typical otter trawl. The otter trawl boardholds the trawl net open by hydrodynamic forces. Such trawlboards are dragged along the seabed and may represent a haz-ard to the pipeline.

Figure 1-2Typical beam trawl gear crossing a pipeline

In beam trawling, a transverse steel beam is used to keep thenet open as shown in Figure 1-2. Beam shoes are mounted ateach end of the beam and represent a substantial hazard topipelines due to their sharp edges and large kinetic energy.In twin trawling the clump weight shown in Figure 1-3 has amass typically in the range of 2 to 9 tonnes, and can causelarger impact energy and pull-over loads than trawl boards.Several designs are used, ranging from a clump of chain tospherical or cylindrical rollers. Twin trawling with clumpweight is currently not used for industrial trawling, and ishence only relevant in consumption trawl areas.

Figure 1-3Typical twin trawling with clump-weight.

Traditionally, pipelines are protected against trawl impact bycoating, gravel or burial. As such protection is expensive; thereis a need for improvement with respect to design methods andrules for trawl gear interference. This RP gives information ondesign methods such that unnecessary conservatism may beavoided. At the same time it intends to give a more uniformsafety level for the potential failure modes.

1.2 Trawling aspectsThe authority requirements with respect to interferencebetween trawl gear and pipeline/ subsea structures vary fromcountry to country. In the Norwegian sector, it is required thatall subsea installations shall not unnecessarily or to an unrea-sonable extent impede or obstruct fishing activities, whereas inother countries the authorities allow non-overtrawlable struc-tures (i.e. by applying safety zones and restricted areas onmaps, or by using guard vessels).Subsea installations attract fish, and hence fishing activity. Agood dialog between the fishing and offshore industries isimportant in order to ensure safe and cost effective operationfor all parties. Examples of elements important to communi-cate are:

— pipelines preferably to be routed outside fishing bankswhenever practical, and thus, designers need importantinformation about such;

— the offshore industry needs information on trawl equip-ment used, to design for appropriate loads and to reducerisk of hooking;

— new trawling equipment should be designed to minimizerisk of hooking pipelines, subsea structures and other sea-bed obstructions; and

— development of new trawl equipment may have impact onexisting pipeline designs.

Trawl velocity and pattern is mainly governed by fish move-ment pattern, sort of fish to catch (swimming speed), and eco-nomic speed of trawl vessels. Therefore, it is not likely that thetrawling velocity will increase significantly in the future.Trawling for prawns is typically performed at 2 - 3 knots (1-1.5m/s), whereas trawling for fish is performed at up to 5 - 6 knots(2.8 m/s average).Presently (2005), the heaviest twin trawl equipment has a typ-ical mass up to 9-10 tonnes and is used in the Barents Sea andoutside Greenland - mainly trawling for prawn in areas withoutoffshore activities. However, trawlers operating in these areasmay also use the same heavy equipment in the North Sea or theNorwegian Sea (i.e. to avoid having two sets of equipment).The weight of the clump weights used is typically 60 - 70 % ofthe total weight of the trawl doors. Trawling along a curved path may cause the trawl equipmentpath to be considerably different from the path of the vessel.Figure 1-4 illustrates a potential scenario where the trawl ves-sel turns around a 500 m radius safety zone and causes thetrawl equipment to follow a path well within the restrictedzone, [9]. It should be noted that trawling inside the safetyzones is not allowed. However experience shows that this mayoccur and should be considered in the design.

Sweepline

Trawl board

Trawl net

Warpline

Trawl net

WarplineBeamBeam Shoe

150 - 350m

Clump Weight

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Figure 1-4Possible trawl vessel and bottom trawl gear positions when fish-ing around platform safety zone.

Trawl interference with pipeline rock-berm and trenches aswell as cuttings deposited during well drilling operations andleft after removal of installations may represent considerableloading (and potential hooking) to trawl equipment. Theseaspects are not covered by this RP.

1.3 Scope and applicationThe objective of this RP is to provide rational criteria and guid-ance on design methods for pipelines subjected to interferencefrom trawling gear; including the impact, pull-over and possi-ble hooking phases. Design criteria are given as well as guid-ance on applicable calculation methods.For pipelines subjected to global buckling, such as:

1) pipelines with release of effective, compressive axial force(buckling) prior to trawling, or

2) pipelines with release of effective, compressive axial forcesimultaneously with trawling, i.e. the trawl load triggersglobal buckling.

Global buckling pipeline response analyses is required in com-bination with the trawl load assessment. The global bucklinganalyses needs to include sensitivity studies of governingparameters such as pipe to soil interaction in accordance withDNV-RP-F110, to establish the proper functional load condi-tion factor.This document is applicable to rigid pipelines with outer steeldiameters larger than 10" (i.e. the smallest pipe diameter testedin model tests have been used as basis for this RP). However,

if measures are taken in the analyses to account for scalingeffects outside of the model test validity range, the methodol-ogy reflected in this RP may still be applicable. Typically, thiswould involve special assessments of soil to pipe interactionfactors (ref. Figure 3-3), global bending stiffness of pipelineand factors related to relative sizes between pipe and trawlequipment.For pipelines with outer steel diameter less than 10", the soilreduction factors based on Figure 3-3 should conservatively bebased on 300 mm outer diameter for impact load calculations.Loads and load effects on flexible pipelines can be determined,provided relevant flexibility characteristics and acceptance cri-teria are specified.Trawl pull-over and hooking may govern the acceptable freespan lengths and gaps. These aspects are covered within thisRP, while the effect of environmental loads on free spans iscovered by DNV-RP-F105.This document does not cover pipeline attachments such asanodes. However, it is envisaged that such equipment isdesigned for trawl loads both with respect to impact and pull-over including abrasion from trawl wire. Additionally, edgesand protrusions and or bolts that may snag trawl nets need tobe avoided.Further, a methodology for qualifying possible DEH cableattached to the pipeline inside a protective structure is given inAppendix A.This RP is intended for use on a world-wide basis. However,the collection of trawl gear data has been carried out for theNorth Sea and the Norwegian Sea. Data is given appropriatefor otter, beam and twin trawling equipment in use in 2005 andexpected for use in the near future in these areas.The following design aspects are considered:

— coating damage due to impact— pipe denting due to impact— overstress due to pull-over or hooking.

This comprises the following topics:

— most critical trawl equipment— frequency of trawl impacts— effective impact energies to be absorbed by the coating

and the pipe— requirements to structural modelling— recommendations for pull-over loads— recommendations for lifting heights due to hooking— acceptance criteria.

Figure 1-5 shows part of a flow chart for a typical pipelinedesign. After deciding on diameter, material, wall thickness,trenching and coating for weight and insulation, the trawlingdesign is performed. It must be emphasised that the trawl pull-over assessment must be based on a realistic estimate of theeffective axial force, and any changes due to sagging in spans,lateral buckling, end expansion, changes in operational condi-tions etc., must be properly accounted for. Unacceptableresults from trawl impact, pull-over and/or hooking maychange the sequence in this design flow chart (Figure 1-5) as itmay have impact on coating and burial of the pipeline.

zone

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Figure 1-5Flow chart for pipeline design

1.4 LimitationsThis RP gives loads and load effects on pipelines due to trawlgear interference. These loads are related to the size, shape,velocity and mass of the trawling gear. This document is basedon data for trawl equipment typically used in the North Sea andthe Norwegian Sea up to 2005, however the methods may beused for other geometrically similar equipment as well.The design criteria and associated set of partial safety factorsare based on the assumption that load effects are obtained froma state-of-the-art FE-analysis or similar calculation methodwhich includes the most important linear and possible non-lin-ear effects of the considered pipeline section.

1.5 Relationship to other rulesThis RP replaces the DNV Guideline number 13 issued in1997, and is a supplement to and complies with the DNV-OS-F101. This document further relates to and harmonises with

the DNV-RP-F110.The NORSOK standard "Subsea Production systems", U-001gives requirements with respect to interference with trawl gear.Generally, this document is in accordance with the require-ments given in the NORSOK standard. However, with regardto trawl impact, pull-over and hooking loads, this RP givesmore precise and detailed estimates for the loads which mayoccur for pipelines while the NORSOK requirements refer totrawl loads on subsea structures. In case of conflict between requirements of this RP and a ref-erenced DNV Offshore Code, the requirements of the codewith the latest revision date shall prevail. Any conflict is intended to be removed in next revision of thatdocument.In case of conflict between requirements of this Offshore Codeand a non DNV referenced document, the requirements of thisCode shall prevail.

1.6 ReferencesThe latest revision of the following documents apply:DNV Offshore standardDNV-OS-F101 Submarine Pipeline Systems, Det NorskeVeritas.DNV Recommended PracticeDNV-RP-C205 Environmental Conditions and EnvironmentalLoads, Det Norske Veritas.DNV-RP-F105 Free Spanning Pipelines, Det Norske Veritas.DNV-RP-F110 Global Buckling of Submarine Pipelines, DetNorske Veritas.Other standardsNORSOK, Standard, "Subsea Production systems", U-001.

1.7 Trawl-pipeline interaction phasesWhen bottom trawl gear is towed across a pipeline, the inter-action may conveniently be divided into two stages: impactand pull-over. As a special case hooking may occur.

— Impact, i.e. the initial impact phase when a trawl board,beam shoe or clump weight hits a pipeline. This phase typ-ically lasts some hundredths of a second. It is mainly thelocal resistance of the pipe shell, including any protectivecoating and/ or attached electric cable protection structurethat is mobilised to resist the impact force.

— Pull-over, i.e. the second phase where the trawl board,beam trawl or clump weight is pulled over the pipeline.This phase can last from about 1 second to some 10 sec-onds, dependent on the water depth, span height, and otherfactors. This will usually give a more global response ofthe pipeline.

— Hooking, i.e. a situation whereby the trawl equipment is"stuck" under the pipeline. This is a seldom occurringaccidental situation where forces as large as the breakingstrength of the warp line may be applied to the pipeline inextreme cases.

An overview of the design process for pipelines with respect tointerference with trawl gear is shown in Figure 1-6. First, trawlequipment characteristics and frequency must be established.Then, analysis and design with respect to impact, pull-over andhooking is conducted. The impact energy to be used in testingof coated pipe sections is calculated. The effect of pull-over isfound through global analysis. The capacity to resist possiblehooking is checked by applying a certain lifting height.

Trench/ Protection /

weight coating / wall thickness

Trenching

Expansion

Check feasibility wrt. installation

Intervention

Burial

Safety evaluations Flow assurance

Material selection Wall thickness design

Trawling

Stability

OK

Yes

OK

OK

OK

No

No

No

Routing

No

No

No

OK

Expansion

Free-span

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Figure 1-6Overview of the design of pipelines with respect to interference with trawl gear.

1.8 UnitsThe international system of units (SI system) is appliedthroughout this document (e.g. length in meters, mass in kilo-grams, time in seconds and force in Newton).

Trawl equipmentSize, mass, frequency, etc.

(Sect. 2.)

Pipeline parametersSafety class, diameter, thickness, coating, etc.

(Sect. 2.)

Impact (Sect. 3)

Pull-over (Sect. 4)

Hooking (Sect. 5)

Simplifiedcalculations

(Sect. 3)

OK

Advanced analysesAbsorbed energy and

dent depths (Appendix A)

Dynamic analysesFE-calculations –

check of limit states (Appendix A)

Lift analysesTrawl board wedged orpenetrated – check of

limit states (Appendix A)

OK

Critical spans(Sect. 5.5 & 5.6)

Height 1 Height 2

OKOK

Verifying coatingTests

(Sect. 7)

OK Successful design

Redesign Redesign

YESNO

NO

YES

NO

NO

YESYES

YESNO

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1.9 Symbols and abbreviationsThe following definitions apply:

Aclump clump weight dimension, gap between roller and pipeline contact point (see Figure 4-1)

Ae cross-sectional area of the steel pipe exposed to external pressure

Ai cross-sectional area exposed to internal pressureAw warp line cross-sectional areaB half height of trawl boardCb coefficient of effective beam trawl mass during impactCF coefficient of pull-over forceCh coefficient of effect of span height on impact velocityCT coefficient for pull-over durationd water depthD steel pipe nominal outside diameterE Young’s modulusEa impact energy due to hydrodynamic added massEDEH impact energy absorbed locally by the DEH cable includ-

ing protection structureEloc impact energy absorbed locally by the pipe shell and coatingEs impact energy due to steel mass of trawl board, beam with

shoe or clump weightfimp trawl gear impact frequencyFb impact force due to trawl board bending actionFsh maximum impact force experienced by the pipe shellfT trawl frequency per considered section, see Sec.6.4.Fp maximum pull-over force on pipe, horizontal directionFz maximum pull-over force on pipe, vertical direction (note

positive down)fy yield stress to be used in design

fy = (SMYS - fy, temp) αUfy, temp derating value due to the temperature of the yield stressg gravitational acceleration

dimensionless height (= (Hsp + OD/2+0.2)/B)h trawl board height (=2B)

dimensionless moment arm, to calculate clump weight pull-over load

Ha height of attachment point of warp to beam trawlHcr critical free span height for hookingHi impact hammer pendulum heightHl lifting height during hookingHp,c permanent dent depth (characteristic value)Hsp span height (pipeline to seabed gap)Ht dent depth (elastic and plastic)I trawl vessel density (annual mean no. of trawlers per unit

seabed area)kb out-of-plane stiffness of trawl boardkc1 stiffness of protection cover for heating cables attached to

the pipeline (when applicable)kc2 coating stiffnesskc3 coating stiffness due to interaction effects between coat-

ing and steel pipeki in-plane stiffness of trawl boardkpb effective bending stiffness of pipe in impact calculation

(time dependent)kps effective soil stiffness applied to pipe in impact calcula-

tion (time dependent)ks local shell stiffness of steel pipekw stiffness of trawl warpL trawl board length, beam length or clump weight widthLclump distance from clump weight interaction point with pipe-

line to clump weight centre of gravity (See Figure 4-1)

H

h′

Lw warp line lengthMt moment resulting from trawl pull-over loadMtd design moment for resulting from trawl pull-over loadma hydrodynamic added mass of trawl board or beammh mass of test impact hammermt steel mass of trawl gearm1 mass of test rig pipe supportm2 mass of test rig floorncap capacity of coated pipe expressed as number of repeated

impacts to failureng number of trawl boards, beam shoes or clump weights per

trawl vesselnmean mean number of impacts of coating before failureN axial force in pipe wall (true force) (tension is positive)OD overall outside diameter of pipeline, including coatingpe external pressurepi characteristic internal pressureRfa reduction factor for impact energy associated with hydro-

dynamic added massRfs reduction factor for impact energy associated with steel massS effective axial force (tension is positive)Su soil undrained shear strengtht pipe wall thickness (steel),

t = tnom - tcorr (i.e. equivalent to t2 in DNV-OS-F101)tnom nominal thickness of the pipe (uncorroded)tcorr corrosion allowanceTp pull-over durationV tow velocity of trawlerαe proportion of pipeline length exposed to trawl interfer-

ence (not buried or protected e.g. by rock cover)αU Material strength factor, ref. DNV-OS-F101:

αU = 0.96, except for pipeline material fulfilling supple-mentary requirements U, where αU = 1.0

δp global deflection of pipe at point of trawl pull-overγA load effect factor, for trawl load classified as accidental

(γA = 1.0)γC load condition factor, ref. DNV-OS-F101γF load effect factor, with the trawl load classified as a func-

tional loadη usage factorϕ prevailing trawl direction relative to the pipeline perpen-

dicularφ soil internal friction angleσ standard deviation for impact test results

DEH Direct Electrical HeatingFE finite elementROV remotely operated vehicleRP Recommended PracticeSCF stress concentration factorSMYS specified minimum yield strengthPolymer materials such as rubber and plasticsCharacteristic load the reference value of a load to be used in the

determination of load effectsLoad effect effect of a single load or combination of loads

on the equipment system, such as stress, strain, deformation, displacement, motion, etc.

Load effect factor the partial safety factor by which the charac-teristic load effect is multiplied to obtain the design load effect.

Industrial trawling catch for industrial production (e.g. oil, meal, animal food)

Consumption trawling catch for food commerce

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2. Trawl Design Data2.1 Types of trawl gearThere are two main categories of bottom trawl gears of concernin the North Sea and the Norwegian Sea; conventional ottertrawl gear, and beam trawl gear used in southern part of theNorth Sea. These differ in the way the trawl net is kept open:

— Otter trawls by use of trawl boards— Beam trawls by use of transverse beams

In addition, the use of pair trawling by two ships and twin rigset-ups has developed in recent years. Such gear must beaccounted for if heavy clump weights are used in order to forcethe trawl net down. The size and design of such clump weightsvaries. Otter trawling in these areas is carried out with a range of shipsizes and types of gear. Typical types of ships and gear are:

— Consumption trawlers which have the largest equipment — Industrial trawlers— Prawn trawlers (operate in the deep water of the Norwe-

gian trench and closer to the coast).

Various trawl board designs exist, however two major types,polyvalent / rectangular and V-shaped boards, are in commonuse. The polyvalent boards have generally been found to givethe highest loads on pipelines. Figure 2-1 shows two typicaltrawl boards used in the North Sea and the Norwegian Sea. Inrecent years, trawl boards with spoilers have been developed,to improve the efficiency of the hydrodynamic force andthereby reduce drag forces and trawler fuel consumption.

Figure 2-1Typical Consumption trawl boards: Polyvalent (upper) andV-board (lower).

The size of the trawl board mainly depends on the type of trawlnet being used. No depth dependency for trawl board sizes andvelocities has been found. It is rather the type and quantity of

fish which governs the trawl type and thereby the board sizeand trawling velocity.A typical beam trawl shoe and its components are shown inFigure 2-2. Beam trawls are developed to catch various speciesof flat fish. They are mainly used on flat, sandy seabed in shal-low waters in the southern parts of the North Sea.

Figure 2-2Typical outline of a beam trawl shoe

Typical clump weights are shown in Figure 2-3 and Figure 2-4.

Figure 2-3Typical clump weight (Bobbin type)

Figure 2-4Typical clump weight (Roller type)

2.2 Basic data requirementsBefore calculating the load and load effects of the pipelinesome basic data with respect to expected trawling along thepipeline route has to be established. These shall include, butnot be limited to, data along the route for:

— trawling category (i.e. industrial or consumption trawlingwith otter, beam trawl gear, or pair / twin trawling withclump weight)

— trawling equipment in use (type, shape, size, mass, trawl-ing speeds)

— expected frequency of trawl gear crossing over the pipe-line; and

Trawl direction

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— expected or possible developments or changes in the fore-seen trawling equipment or frequency (e.g. new equip-ment, larger vessels, increased frequency, etc.).

The route should be divided into subsections categorised in asuitable manner with respect to the above factors.Data shall be obtained from the relevant authorities. Thedesign process shall ensure that the collected data are relevantand up to date.If development of trawl equipment deviates from designassumptions, the need for reanalyses to account for these (e.g.trawl equipment type, mass, velocity and frequency) has to beconsidered.

2.3 Most critical trawl gearIt should be observed; when considering the most criticaltrawling equipment, velocities, impact frequencies and area ofapplication; that these parameters are subject to change. As anexample, the largest trawl board in the North Sea and the Nor-wegian Sea has increased from about 1 500 kg in the late 70'sand 80's to 4 000 kg in 2005. In the Barents Sea trawl boardsup to 6 000 kg are currently being used.Development of fishing technology may open new areas forfishing, while international and national laws and regulationsmay introduce changes in fishing activity in certain areas. It istherefore essential to establish relevant data for trawl equip-ment and to assess possible future development and its impacton trawl-pipeline interaction.Table 2-1 lists the appropriate data for the largest trawl boards,beam trawls and clump weights in use in the North Sea and theNorwegian Sea in 2005. However, the following area specifictrawl data needs to be investigated for each project:

— the types and maximum size of trawl gear normally usedin the area

— future trends (new types, mass, velocity and shape)— the trawling frequency in the area.

2.4 Trawl gear impact frequencyDue to evolution of fishing equipment and change in fishingstocks, the trawling frequency may change significantly duringthe lifetime of the pipeline. The following aspects should beaddressed to obtain a good estimate of the frequency of inter-ference (hereafter termed the trawl gear impact frequency)between the pipeline and trawl gear:

— fishing vessel density in the relevant area— prevailing trawling direction relative to the pipeline— distribution of different trawl equipment and sizes

If this information is not available, the highest frequency classaccording to Section 6 shall be used.An estimate of the trawl gear impact frequency, fimp, is:

where ng is the number of trawl boards, beam shoes or clumpweight for each vessel, I is the expected trawler density (i.e.annual mean number of trawlers per unit seabed area), V is thetrawling velocity, αe is the proportion of the pipeline lengthexposed to trawl loads, and ϕ is angle of the prevailing trawlingdirection relative to the pipeline perpendicular. If sufficientinformation is available a distribution function for the trawlingdirections can be applied instead of the cos ϕ term. ϕ applied inEq. (2.1) should not be taken larger than 75°.

3. Impact3.1 IntroductionThe impact loads are associated with the transfer of kineticenergy from the impacting trawl board, beam shoe or clumpweight to the pipe, its coating, possibly DEH cable protectionstructures and the soil. Generally the time of energy transfer isso short that most of the energy transferred is absorbed as localdeformations. For smaller diameter pipelines some energy isalso absorbed by global deformation of the pipe and in the soil.The impact assessment covers the following main steps:

— calculate the impact energy level the system will absorbfrom considered trawl gear interference, as basis for theimpact test specifications

— calculate the pipe steel wall capacity with respect to spec-ified denting acceptance criterion.

The methods included here are based on the theoretical workpresented in [1] and [4].

3.2 Impact energyThe impact direction and the amount of energy transferred tothe pipe coating depend in general on the shape of the front ofthe trawl board, clump weight or beam shoe, on the pipe diam-eter and span height and on the direction of travel relative tothe pipeline.The impact energy constitutes effective masses with an effec-tive velocity. The total effective mass consists of the steel massof the board/beam/clump and the hydrodynamic added mass,including mass of entrained water. The effective velocity maybe taken as the towing velocity component normal to the pipe-line.The effective masses and velocity may be obtained throughsimulation of the trawl gear-pipeline interaction. AlternativelyTable 3-1 gives design data which can be applied if no moredetailed information is available. It is emphasised that trawlgear data differs per location, and may change in future. There-fore, area specific trawl data that accounts for future develop-ment should be established.

3.3 Energy dissipationThe kinetic energy of the trawl gear may partly or entirely bedissipated during the impact by:

— deformation of trawl equipment— deformation of protection cover for attached heating

cables etc.— deformation of coatings (i.e. corrosion-, weight-, thermal

insulation- and / or field joint coating)— elastic deformation and possibly plastic denting of pipe

steel wall— global deflection of the pipeline, including soil friction

effects; and / or— deformation of soil.

Each of these elements needs to be represented by equivalentcharacteristics (spring stiffness, mass etc.), when applying theadvanced impact assessment method given in Appendix A.The spring characteristic should be based on tests or equivalent

Table 2-1 Data for largest trawl gears in use in the North Sea and the Norwegian Sea in 2005

Consump-tion

Industrial Beam Clump weight

Mass [kg] 4 500 5 000 5 500 9 000Dimension Lxh [m] 4.5 × 3.5 4.9 × 3.8 17.02) 1)

Trawl velocity [m/s] 2.8 1.8 3.4 2.81) Typical dimension of the largest clump weights of 9 tonnes are

L = 4 m wide (i.e. length of roller) by 0.76 meter diameter cross section. For smaller size roller type clump weights (i.e. 3 500 to 6 000 kg), the width L is typically 3.2 m, whereas the roller diameter is unchanged.

2) Beam trawl length (i.e. distance between outside of each shoe)

(2.1)ϕ⋅α⋅⋅⋅= cosVInf egimp

DET NORSKE VERITAS


analysis to establish the representative load displacement char-acteristics - including nonlinear effects. For cable protectioncovers, the load / displacement level at which load is trans-ferred directly to the cable needs to be accounted for.The impact energy assessment gives the total energy that theconsidered system will absorb, which is used as basis for the

energy level to be applied in the impact testing. Elements withlow stiffness, such as field joint coatings and cable protectioncovers may increase the total energy absorbed by the pipeline,although the energy absorbed by the pipe steel wall maydecrease.

Figure 3-1Ch coefficient for effect of span height on impact velocity

Table 3-1 Design parameters for trawl gear impactParameters Consumption Industrial Beam Clump

weightShape of board Polyvalent &

rectangularV-board

Direction of impact ϕ deg 45 18 0 0 0Effective impact velocity: m/s 2.8 Ch 1) 2.8 Ch 1) 1.8 Ch 1) 3.4 2.8Steel mass: mt kg 4500 4500 5000 5500 9000In plane stiffness: ki MN/m 500 500 500 4200 2)

Bending board stiffness: kb MN/m 10 10 10Hydrodynamic added mass: ma kg 2.14 mt 1.60 mt 2.90 mt

5)2.14 mt

15004) 3140 3) 4)

1) The factor Ch is given by Figure 3-1.2) The largest clump weights used per 2005 are the "roller type", see Figure 2-4. The specified clump weight stiffness is conservatively

based on the stiffness of the corner plate. Alternatively, more advanced dynamic analyses with less conservatism can be performed, accounting for flexibility of the clump weight structure absorbing some amount of energy (if available), and considering the two scenar-ios, see Figure 3-2:

a) Base case: Trawl direction normal to the pipeline, single point impact at mid-span of clump weight. Effective clump weight mass is equal to the total steel mass.

b) For locations with prevailing trawl direction inclined to the pipeline, impact energies may be distributed into two frequency classes, one normal to the pipeline and one oblique to the pipeline axis. The latter implies impact at clump weight corner, causing in-plane rotation of the clump weight. Effective clump weight mass may then be approximated as ½ the total steel mass. For impact directions oblique to the pipeline, the velocity component normal to the pipeline may be used.

3) Based on sea water volume of roller cylinder (diameter 0.76 m, and length 3.14 m), calculated in accordance with DNV-RP-C205. Factor to account for seabed proximity is taken as 2.29 and reduction factor to account for length of cylinder is 0.8. Note, for reference an equiv-alent clump weight of mass of 6 000 kg has an estimated hydrodynamic added mass and mass of entrained water of 2 890 kg (i.e. based on roller diameter 0.76 m, length 2.34 m, and mass of entrained water of 890 kg).

4) Includes mass of water entrained in the hollow section. For the 9 000 kg clump weights, the mass of the entrained water estimated as 465 kg (ref. manufacturer). For an equivalent 6 000 kg clump weight, the mass of the entrained water is specified as 890 kg.

5) For V-boards up to 1 500 kg mass, hydrodynamic mass to be taken as 2.9 mt, for larger polyvalent boards 2.14 mt may be assumed.

0.8

0.9

1.0

1.0

Span Height (m)

ChV-board

Polyvalent board(consumption)

2.00.3

DET NORSKE VERITAS


Figure 3-2Relevant clump weight impact configurations

3.4 Simplified response calculations

3.4.1 GeneralThe impact assessment methodology given in the followingapplies for steel pipelines (i.e. bare pipes, painted pipes andpipes with thin corrosion coating and / or with concrete coat-ing). For other configurations (e.g. pipeline with other coat-ings, or pipelines with piggy-back electrical cable withprotection structure) special assessments are required, seeAppendix A. The following simple, conservative method may be used tocalculate the energy absorbed by the pipe locally, assumingthat the pipe deforms locally by indentation, and that all theimpact energy is absorbed through such a deformation. Correc-tion factors for energy absorbed by global pipeline deforma-tion and in the soil may be found (ref. Rfs and Rfa) in Figure 3-3. Alternatively, the pipeline response may be assessedthrough more advanced analyses, as described in Appendix A.

3.4.2 Trawl boardThe impact energy associated with the steel mass of the trawlboard is:

where mt is the trawl board steel mass. Rfs is a reduction factordepending on the outer pipe diameter and given in Figure 3-3for bare steel pipes and pipes coated with concrete. For softercoatings, e.g. polymers, Rfs should be assessed but can con-servatively be set to 1.0.Ch is span height correction factor for the effective pull-overvelocity, see Figure 3-1.

Guidance note:For pipelines with outer diameter less than 300 mm, the reduc-tion factors given in Figure 3-3 for 300 mm could conservativelybe used, unless otherwise documented.

---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---

Pipeline

Clump weight

Warp line

Seabed

b) Clump weight roller hit partly submergedand / or small diameter pipeline

c) Impact configuration at spans, fordifferent pipeline diameters

Pipeline

Clump weight, 9 tonnes, 2.8 m/s

Sweep line

Centre warp line bracket of clump weight,hitting perpendicular on pipeline – i.e. noreduction in impact energy due to rotation ofclump weight.

Sweep line

Impact scenario 2:

PipelineOD

Clump weight

Warp line

Impact energy absorbed bypipeline depends on pipelinediameter relative to clump weightgeometry.

a) Clump weight front frame plate (typ. 40-50mm thick) with warp line pad-eye hit thepipeline

Pipeline

Sweep line

Sweep line

Warp line

Clump weight corner hitting the pipeline,giving reduced impact energy (i.e. due toreduced effective mass by approx. 50 %,).

Clump weight, 9 tonnes, 2.8 m/s

Impact scenario 1:Warp line

b) Clump weight roller hit partly penetrated into soil and/or small diameter pipeline.

(3.1)221 )( VCmRE htfss ⋅⋅=

DET NORSKE VERITAS


Figure 3-3Reduction factors for concrete coated and bare steel pipes.

The main contribution of the hydrodynamic added mass is inthe direction perpendicular to the board and is activatedthrough lateral bending of the board. The impact force associ-ated with the hydrodynamic added mass of the trawl board maybe estimated as:

The associated impact energy from added mass of the trawlboard is:

where:

where ma is the trawl board added mass, t the steel wall thick-ness (i.e. to be taken as the nominal wall thickness minus thecorrosion allowance) and kb the lateral bending stiffness of theboard. αU is the material strength factor, ref. DNV-OS-F101.fy,temp is the temperature derating value of the yield stress.αU = 0.96, except for pipeline material fulfilling supplemen-tary requirements U, where αU = 1.0. Rfa is a reduction factor depending on the pipe diameter and thesoil type which is given by Figure 3-3 for bare steel pipes andpipes coated with concrete. For softer coatings, e.g. polymers,Rfa should be assessed but can conservatively be set to 1.0.

Guidance note:The pipe wall thickness to be applied is t = tnom - tcorr, howeverif more detailed knowledge on corrosion mechanism exists, e.g.that corrosion is limited to grooves at 6 and/ or 12 o'clock posi-tion and is concluded to cause minor reduction in trawl loadcapacity, applying t = tnom may be acceptable.


Appropriate parameters are given in Table 3-1. Eq. (3.5) isconservatively based on a relationship between the impactforce and associated indentation of the pipe representative fora bare steel pipe:

Here, Fsh is the impact force experienced by the pipe shell andHt is the dent depth (i.e. both elastic and permanent).A conservative estimate of the kinetic energy absorbed bylocal deformations of the coating and the pipe wall is found bytaking the maximum of Es and Ea:

3.4.3 Beam trawlFor beam trawls, the impact energy absorbed by the pipe andits coating may be calculated as:

where Cb is a factor taking into account the effective mass andmay conservatively be set equal to 0.5 if a more precise esti-mate is not available. mt is the steel mass of the beam trawlinclusive shoes, and ma is the hydrodynamic added massincluding the mass of water entrained in the hollow beam.

3.4.4 Clump weightsFor clump weights, the total absorbed energy can be calculatedas:

0

0.2

0.4

0.6

0.8

1

1.2

300 400 500 600 700 800 900 1000 1100Outer Diameter [mm]

Red

uctio

n fa

ctor

, Rfs a

nd R

fa

Added mass, Rfa

Steel mass, Rfs

sand emb (φ≤35o) or clay (Su=15 kPa)

clay (Su=50kPa)

sand (φ=50o) or clay (Su=35 kPa)

sand (any friction angle)or clay (Su=50kPa)

or free spanning pipes

Notes to the reduction factors:1. For clay stiffness values aboveSu=50 kPa and on rocky seabedthe reduction factor is to be takenas unity (1.0)2. φ=35o (for sand) or Su=5 kPa(for clay) is to be used as aminimum for the reduction factor.The reduction factor shall not betaken as less than 0.1.3. For soil values inbetweenlinear interpolation shall be used.

clay (Su≤5kPa) or free spanning pipes

(3.2)

(3.3)

fy = (SMYS - fy, temp) αU (3.4)

bahb kmVCF ⋅⋅⋅=

221

32

3

)(75

)(2 VCmtf

FRE hay

bfaa ⋅≤

⋅⋅⋅=

(3.5)

(3.6)

(3.7)

(3.8)

21

23

5 tysh HtfF ⋅⋅⋅=

⎭⎬⎫

⎩⎨⎧

=a

sloc E

EE max

221 )( VmmCRE atbfsloc ⋅+⋅=

221 )( VmmRE atfsloc ⋅+⋅=

DET NORSKE VERITAS


where mt is the dry steel weight of the clump weight, and ma isthe hydrodynamic added mass including the mass of waterentrained in the hollow sections.The added mass can be estimated as the mass of the displacedsea water volume, multiplied by a factor 2.29 for proximity tosea bed and a factor 0.8 for limited length, according to DNV-RP-C205. The mass of entrained water is given in Table 3-1.The effective impact mass (mt + ma), the local stiffness and thein-plane stiffness of the clump weight depend on impact con-figuration as shown in Figure 3-2. The clump weight mass andstiffness given in Table 3-1 are conservative. Less conserva-tive values may be adopted based on dynamic simulations ortests of actual configuration.Two impact scenarios are considered relevant for clumpweights (see Figure 3-2):

— Trawl direction normal to the pipeline, with the clumpweight hitting with its total mass at centre of the clumpweight. Typically, the warp line pad-eye protrudes causinga single point contact. If there are no protrusions, theimpact load transfer will be distributed over a larger areaand hence becomes less critical.

— Trawl direction at an inclined angle to the pipeline, withthe corner of the clump weight hitting the pipeline. Theimpact energy will be reduced from rotation of the clumpweight.

— For scenarios where the pipeline interferes with the rollerdirectly (e.g. for partly buried and / or smaller diameterpipelines, see Figure 3-2 b) the clump weight will roll overand cause smaller loads to the pipeline, than for interfer-ence with the structural frame of the clump weight as illus-trated in Figure 3-2 a and c.

The governing impact scenario depends on type of clumpweight, relative height of clump weight versus pipeline diam-eter, trawl direction, span height etc.The clump weight impact specification given in Table 3-1 arebased on conservative estimates. Less conservative values maybe adopted, if documented by representative tests or dynamicsimulations.

3.4.5 Pipe shell indentationIf no detailed relationship between the impact force and asso-ciated indentation of the steel pipe wall is available, the perma-nent indentation of the pipe shell caused by the impact may beestimated as:

where Hp,c is the estimated permanent plastic dent depth and:

Eq. (3.9) and (3.10) are valid for bare steel pipes. Pipes withthick polymer coating will in general be subjected to smallerimpact loads but absorb more energy. The latter is caused bythe significant energy absorption in the coating. Therefore, theabove simplified approach may give non-conservative resultsfor such coatings, and the advanced method given in AppendixA should be applied. Similar comments apply to DEH cableprotection structures.This simplified analysis will lead to increasingly conservativeresults for smaller and lighter pipes. In this case the advancedassessment (Appendix A) is recommended.

4. Pull-over4.1 IntroductionPull-over analysis deals with the global response of the pipe-line as the trawl gear is pulled/ forced to cross over the pipe.During this phase, the pipeline may be subjected to relativelylarge horizontal (lateral) and vertical forces.The pull-over calculation procedure for trawl boards is basedon both experimental [3, 5, 6] and theoretical work [7, 8], thebasis for beam trawls is tests [1, 2], while the basis for theclump weights is established as part of specific field develop-ment projects in the Norwegian Sea.

4.2 Structural modellingThe structural behaviour of the pipeline during pull-over shallbe evaluated by modelling a sufficiently large part of the pipe-line, the seabed and other supports and performing a structuraldynamic analysis by applying the pull-over load (static analy-sis is likely to be non-conservative).The most critical position for trawl pull-over depends on:

— length, height and shoulder supports of free spans;— effective axial force;— lateral curvature of the pipeline; and— lateral or axial supports or restraints.

The pull-over load for trawl boards and clump weights shall beapplied as a single point load while the pull-over load for beamtrawls may be applied as two concentrated loads at a distanceequal to the width of the trawl beam and with half the pull-overforce at each position.All relevant non-linear effects shall be taken into account in theanalysis of trawl gear pull-over. These may be, but notrestricted to:

— buckling effects caused by effective axial force;— large displacements including geometrical stiffness;— soil resistance; and— non-linear material behaviour

The effective axial force must be accounted for:

where pi and pe denote the internal and external pressures andAi and Ae the corresponding cross-section areas of the steelpipe exposed to internal and external pressure, respectively.For more details, see DNV-OS-F101.The boundary conditions at the ends of the pipeline modelshall be sufficient to represent the pipe-soil interaction and thecontinuity of the whole pipeline length. In case of buckling itis important to model a sufficient length of the pipeline (anchorlength) or by alternative boundary conditions allow the buck-led section to be exposed to the potential axial feed-in.Due to erosion/scouring, free spans may develop over time andchange the pipe-soil interaction. This must be accounted for inthe analysis/modelling of trawl pull-over.

4.3 Pull-over loads for trawl boards and beam trawlsPull-over loads may be assessed through model tests or numer-ical simulations. Alternatively, if the flexibility of potentialfree-spans is not dominating, the method given below may beused for 10" to 40" pipelines.The maximum horizontal force applied to the pipe, Fp, is givenby:

(3.9)

(3.10)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅⋅

⋅⋅−⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛

⋅⋅= 2/3

2

2/3,5

005.05 tf

DFtf

FHy

sh

y

shcp

3/132275 )( tfEF ylocsh ⋅⋅=

(4.1)eeii ApApNS +−=

DET NORSKE VERITAS


Trawl boards:

Beam trawls:

where kw is the warp line stiffness and V the trawling velocity.mt is the steel mass of board or beam with shoes, and ma is thehydrodynamic added mass and mass of entrained water. Theempirical coefficient CF is a function of trawl gear type andsome geometrical parameters characterising the trawl-pipelineinteraction.For trawl boards this coefficient is:For polyvalent and rectangular boards:

For V-shaped boards:

and is a dimensionless height:

Here, Hsp is the span height (negative for partly buried andtrenched pipelines), OD the pipe outer diameter includingcoating and B is half the trawl board height. The span heightshould represent a conservative estimate to account for uncer-tainties (e.g. survey inaccuracy).Trawl loads on trenched or partly buried pipelines have notbeen covered by the model tests used as the basis for this RP.For partly buried and or trenched pipelines, this may be simu-lated by specifying a negative span height, Hsp, in the pull-overload calculations.

For beam trawls:With hoop bars:

Without hoop bars:

CF shall be taken as for beam trawls without hoop bars if it isnot assured that all beam trawls in the relevant area have suchbars installed. If no better information is available, the attach-ment point of the warp line, Ha, may be set to 0.2 meters.The warp stiffness, kw, is estimated as kw = EAw/Lw, whereEAw is the axial stiffness of the warp and Lw its length. If no

better information is available, the warp stiffness of one singlewire (typically 32-38 mm diameter) may be calculated as:

where Lw is the length of the warp line in meters. Typically, thewarp line length is 2.5 to 3.5 times the water depth. The wirelength is relatively longer in shallow water (i.e. 2.5 times is fordeep water applications). It should be considered that two orthree 32 mm diameter wires are commonly used as warp linesfor beam trawls.The warp line stiffness given above is based on the assumptionof a straight wire between the bottom trawl gear and the vessel.In case of a catenary shaped warp line, the stiffness will belower which again will cause a lower pull-over force. In casesuch a reduced stiffness shall be applied, the weight of thewire, the warp line force and the drag load caused by combina-tion of trawling velocity and current have to be accounted for.For trawl boards the maximum vertical force acting in thedownward direction can be estimated as:Polyvalent and rectangular boards:

V-shaped boards:

This downward acting force may be assumed to have the sameforce-time history as the corresponding horizontal force.The pull-over forces given by Eq. (4.2) to (4.11) are valid forone trawl board while for beam trawls these equations give thetotal pull-over force from both beam shoes.

4.4 Pull-over loads for clump weightsAs for trawl boards and beam trawls the pull-over loads maybe assessed through model tests or numerical simulation.Alternatively, if the flexibility of potential free-spans is notdominating, the method given below may be used for pipes of10" to 40" diameter (i.e. range of dimensions used in the modeltests to derive pull-over loads from clump weights) as long asthe clump weight is of the sphere/bobbin or roller type.

Figure 4-1Clump weight interaction with pipeline

(4.2)

(4.3)

(4.4)

(4.5)

(4.6)

for (4.7)

for (4.8)

2/1)( wtFp kmVCF ⋅=

[ ] 2/1)( watFp kmmVCF ⋅+⋅=

)1(0.8 8.0 HF eC ⋅−−⋅=

)1(8.5 1.1 HF eC ⋅−−⋅=

H

BODH

H2.02/sp ++

=

⎪⎩

⎪⎨

⎧−=

0.3/0.6

0.4

aF HODC

3/3/22/

>

<<

<

a

a

a

HODHODHOD

⎪⎩

⎪⎨

⎧−=

5.3/5.10.8

0.5

aF HODC3/3/22/

>

<<<

a

a

a

HODHODHOD

[N/m] (4.9)

(4.10)

(4.11)

ww L

k7105.3 ⋅

=

)8.02.0( 5.2 Hpz eFF ⋅−⋅+=

pz FF 21=

L clump

Hsp

Clump weight Centre of gravity

OD

mt g

Seabed

Fp

Fz

Warp line

Pipeline

A clump

DET NORSKE VERITAS


The estimate for the maximum horizontal pipeline pull-overforce from clump weights is given below:

where (see Figure 4-1) OD is the outer diameter of pipe includ-ing coating, and Lclump is the distance from reaction point tocentre of gravity of the clump weight, mt is the steel mass andg is the gravitational acceleration.The distance from the reaction point to the centre of gravity ofthe clump weight can be taken as:

Note that Lclump is constant, and not to be scaled for other sizesof clump weights, unless more specific data is available.Further, the model tests are base on roller type clump weightdrum diameter of 0.76 m. To account for larger drum diame-ters, this may be approximated by linear scaling:Roller type clump weight:

Bobbin type clump weight:

where 0.32 and 0.28 are the un-scaled lengths. Aclump, for theroller- and bobbin type clump weights respectively (seeFigure 4-1), whereas Ddrum and Aclump represents the newdimensions for the clump weight with larger drum/ spherediameter.The maximum upward (vertical) pull-over force for clumpweights is estimated by:

While the maximum downward force (i.e. negative sign)becomes:

In each case it should be considered if the upward or the down-ward force will give the most critical load combination.

4.5 Pull-over duration Trawl boards and beam trawlsThe force-time history of the horizontal and vertical forceapplied to the pipe is shown in Figures 4-2 and 4-3 for trawlboards and beam trawls, respectively.

Figure 4-2Sketch of force-time history for otter trawl board pull-over forceon pipeline. (Applies for both lateral and vertical pull-over forces,Fp and Fz).

Figure 4-3Sketch of force-time history for beam trawl pull-over force onpipeline

The total pull-over time, Tp, is given by:

Note that the same coefficient, CF, is used as for the forces andis given by Eq. (4.4) to (4.8). δp is the displacement of the pipe at the point of interactionwhich is unknown prior to response simulations. Therefore,the value of δp/V must be assumed (e.g. as CT CF (mt/kw)1/2/10)and may be corrected after response simulations in some sortof iterative approach. However, the response has normallybeen shown to be rather insensitive to realistic values of δp.The coefficient for the pull-over duration, CT, is given as:

The fall time for trawl boards may be taken as 0.6 seconds,unless the total pull-over time given by Eq. (4.20) is less thanthis, in which case the fall time should be equal to the total timebut still allowing for some force build-up say 0.1 second.Care should be taken in case of very short pull-over durations.A sensitivity check with respect to the duration is recom-mended, especially if the duration is close to half the naturalvibration period of a span.

4.6 Pull-over duration - clump weightThe model tests shows that the clump weight can be repre-sented as a quasi-static load, and dynamic loading effects arenot significant during the pull-over. First the clump weight

(4.12)

(4.13)

- roller type (4.14)

- sphere/bobbin type (4.15)

(4.16)

(4.17)

(4.18)

(4.19)

( )65.0

'8.119.3−

⋅−⎟⎟⎠

⎞⎜⎜⎝

⎛⋅−⋅⋅⋅=

clump

htp L

ODegmF

( ) clumpsp LODHh /+=′

[ ]mLclump 7.0=

[ ]mLclump 55.0=

[ ]mAD

L clumpdrumclump 32.076.05.0

5.07.0

+⋅

+⋅⋅=

[ ]mAD

L clumpdrumclump 28.053.05.0

5.055.0

+⋅

+⋅⋅=

gmFF tpz ⋅⋅−= 4.03.0

gmFF tpz ⋅⋅−= 1.11.0

(4.20)

CT = 2.0 - trawl boards (4.21)

CT = 1.5 - beam trawls (4.22)

Force [N]

P- board

V - board

Fp , F z

T p

Time [s]0.2s 0.6s

Tp Time (s)

Fp

Force (N)

0.7Fp

0.6T

VkmCCT pwtFTp /)/( 2/1 δ+⋅=

DET NORSKE VERITAS


stops in the collision, then the warp line is tightened until theclump weight is rotated over the pipeline. However, the pipe-line response may be dynamic, e.g. if global buckling is trig-gered. Thus, the following parameters are governing for thepull-over duration.

— trawl velocity— pipeline induced movement at interaction— warp line stiffness— clump mass

The pull-over duration of a bobbin and roller type clumpweight can be calculated as:

where, as for trawl boards and beam trawls, δp is the displace-ment of the pipe at the point of interaction which is unknownprior to response simulations. Therefore, the value of δp/Vmust be assumed (e.g. δp/V = 0.1Fp/ (kw · V)) and may be cor-rected after response simulations in some sort of iterativeapproach.The force-time history of the pull-over force for clump weightsis shown in Figure 4-4 below.

Figure 4-4Sketch of force-time history for clump weight pull-over force onpipeline (Applies for both lateral and vertical pull-over forces, Fpand Fz).

4.7 Response calculations Inertia forces reduce global response but may cause localisa-tion of bending, such that it is uncertain whether neglectinginertia will be conservative or not. Therefore it is recom-mended to conduct the pull-over analyses as dynamic analyses. Pipeline deflection and resulting bending moment from pull-over loads depend on axial and lateral soil resistance:

— a lower bound axial friction is conservative with respect topipeline displacement

— a lower bound lateral soil resistance may be conservativewith respect to bending moment from the pull-over forcealone, but may be non-conservative if the displacement isdominated by the compressive axial force in the pipeline.

A sensitivity study needs to be performed, analysing the pipe-line response from pull-over loads considering both upper- andlower bound lateral soil resistance, combined with the lowerbound axial friction. The global analysis needs to include sen-sitivity studies of governing parameters such as pipe to soilinteraction, to establish the proper functional load conditionfactor.The soil resistance applied in the structural model shall takeinto account the short duration of the pull-over load.In case of pipeline sections exposed to global buckling due totemperature and/or pressure loads, trawl pull-over may triggerlateral buckling, and reference is made to the design method-ology and design criteria in DNV-RP-F110.

The pull-over analysis shall consider effects from subsequenttrawl pull-over considering the deformed shape from a previ-ous pull-over and the most critical direction for subsequentcrossings. The number of subsequent pull-over events shallconsider the expected number of trawl gear interactions overthe design life for the relevant section.

5. Hooking5.1 IntroductionTrawl gear hooking of pipelines, such that the trawl gear getsstuck, is a rarely occurring event. As an example there werereported a total of 7 events of trawling gear hooking on theNorwegian Continental Shelf related to pipelines from 2000 to2003. In this context, hooking is defined as the condition where thetrawl gear is wedged under the pipeline, the trawler is forced tostop, back up, and attempt to free the gear by winch in the warpline (usually lift directly up).The most likely scenarios for hooking, according to observa-tion from small scale and field tests are:

— a de-stabilised trawl board (dragged on its back along theseabed) approaches a pipeline on the seabed or a free span-ning pipeline with a small gap. The trawl board may digunder the pipeline and get hooked.

— the crossing angle with the pipeline is less than 45° and thepipeline is free spanning. The warp line lifts the trawlboard off the seabed, it slides along the pipeline, becomesde-stabilised, turns over and slides underneath the pipelineuntil it gets wedged at the span shoulder.

For this reason free spans represent an increased risk of hook-ing. However, even for pipelines resting on the seabed, hook-ing cannot be ignored. A trawl-board may become unstableand partly penetrate underneath the pipeline. Such a scenario ismost likely when the board is towed in a direction almost par-allel to the pipeline or there are several pipelines or other sea-bed obstructions in the area. A typical hooking scenario at afree span is illustrated in Figure 5-1.The typical response of trawlers experiencing hooking can bedescribed as:

— Initially the trawler moves with a constant velocity. Thetension from the warp lines onto the drum of the trawlwinches is relatively even. Brakes lock the trawl winchdrum. The brakes are pre-set to avoid damage to the trawlin case it gets fastened.

— If the trawl gear hooks the pipeline, the winch starts pay-ing out with the pre-set tension governed by the brakes.

— The trawler reduces/reverses the propulsive force.— The trawler tries to unfasten the equipment by pulling with

the winch power from different directions.— If this does not work, the trawler may haul the warp lines

and pull until something breaks or the gear comes loose.This is provided that the winch has sufficient power orstructural strength and brake power if the ship motion isused for jerking. The jerking can be caused by wave actionor by propulsion.

This implies that the pipeline structural integrity has to be con-sidered for large hooking loads in different directions. Usuallythe most extreme load will be vertical lifting until the trawlgear is loose or the capacity of the lifting wire is reached.Two hooking conditions are considered:Part penetration: where a part of the considered trawl gearcomponents (trawl board, clump weight or beam) is stuckunderneath the pipeline. For trawl boards this may occur for allspan heights, also for pipelines resting on the seabed, while for

(4.23)VVkFT pwpp /)/( δ+⋅=

Force [N]

Fp

Tp

Time [s]0.2s

0.5F p

0.6s

DET NORSKE VERITAS


clump weights and beam trawl a certain minimum span gap isrequired.Wedged is defined as the condition when the trawl gear com-ponents have crossed under the pipe in a free span and is stuckat the opposite side of the pipeline with the warp line under thepipe. This is considered as a scenario with a very low likeli-hood and is only relevant for trawl boards. A certain criticalspan height is required for this to happen.

5.2 Structural modelThe integrity of the pipeline when exposed to hooking is per-formed by static finite element analyses, lifting the pipeline atthe hooking point to the critical height in accordance with Sec-tion 5.5 and 5.6. The element type, pipeline model length andboundary conditions requirements are the same as specified inSection 4.2.

5.3 SnaggingAs for subsea structures it must be ensured that no protrudingcomponents or details exists that parts of the trawl gear such asthe ground rope of trawl net can snag on. Examples may bebolted flanges, Tees, valves, etc. These need to be protected /covered in order to avoid large snagging loads and in order tobe overtrawlable.

5.4 Critical span heightTrawl boards may become wedged for a free span heightexceeding:

The critical span height with respect to board wedging is pro-portional to the board size. Hence, small boards are equallyexposed to hooking as larger ones, they are more frequentlyused and give a smaller critical span height for wedging.Part penetration is relevant for clump weights and beam trawlsprovided the span height exceeds:

In case of beam trawling, span heights above the critical heightare normally not allowed since hooking may lead to excessiveloads due to the potential lever effect of the long beam.

Guidance note:Rock dumping underneath the pipeline has been used to reducethe span height below the critical value to mitigate potentialwedging. It is assumed that the rock berm must have a certainwidth related to the length of the trawl board to give the intendedeffect.


5.5 Part penetrationIf the maximum span height is below the critical height givenin Section 5.4, the pipeline should be analysed for a static lift-ing height Hl, possibly limited by the maximum warp force

(Section 5.7). Hl may be estimated as:

In the lifting height expression for trawl boards above, the dis-tance from the front to the first warp line attachment pointshould be used as "B", if this distance is larger.If detailed dimensions about the trawl boards used in the areaare not known, use of half the trawl board height, B, is assumedadequate. The lifting height expression for clump weights is a ratio of theclump weight width, L.For beam trawls, span heights exceeding critical height is notallowed (ref. Section 5.4), and lift height from part penetrationis hence not relevant.

5.6 WedgedIf the maximum span height exceeds the critical height givenin Section 5.4, the pipeline should be analysed for an increasein static lifting height of:

5.7 Maximum warp line forceThe maximum warp line pull may be limited by:

— the strength of the warp line— the braking capacity of the winches— the maximum winch power

The pipeline can be subjected to the maximum warp line forceif the trawl gear does not loosen itself, e.g. by lifting the pipe-line, before this force level is reached.The breaking strength of a typical warp line is approximately400 kN (32 mm diameter). However, double and triple warpwires (up to 3 × 38 mm diameter) are used for the heaviesttrawl gears giving a total breaking strength of 800 - 1 200 kN.Note that loads exceeding the winch power may be achievedby locating the trawler just over the point where the trawl gearis hooked, hauling the warp line until it forms a straight, verti-cal line, lock the winch and using wave motion and/or propul-sion force to increase the warp line force.

5.8 Response calculationsThe hooking response can be calculated by a static analysisapplying the maximum lifting height as a prescribed displace-ment. The maximum warp line force may limit the relevantlifting force used in the analysis.For calculation of the response during hooking relevant andconservative models shall be applied. If simpler methods areused, the conservatism in the results should be documented.Assessment of hooking must consider the effect of anyrestraint including rock berms, tie-in points, subsea structuresand other types of restraint.

- trawl boards (5.1)

[m] - clump weights (5.2)

[m] - beam trawls (5.3)

BHcr 7.0=

7.0=crH

5.0=crH

- trawl boards (5.4)

- clump weights (5.5)

(5.6)

ODBH l ⋅−= 3.07.0

LH l 2.0=

BH l =

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Figure 5-1Sketch of typical hooking scenario in a free span

a) trawl board approaches pipeline span at a skew angle,

b) warp line comes in contact with pipeline and lifts board off seabed, boardturns over

c) board slides along pipeline with its front underneath the pipe until stuckat the span shoulder.

6. Acceptance Criteria6.1 GeneralThe following specifies acceptance criteria for design or inoperation assessments to prevent damage due to trawl interfer-ence. These criteria are based on DNV-OS-F101 wherein clas-sification of safety classes is used to define acceptance criteria.In DNV-OS-F101, the pipeline loads are classified into differ-ent load categories to relate the load effect to different uncer-tainties and occurrences. Trawl loads are classified as an"Interference" load. The criteria are further differentiated withrespect to the trawl gear impact frequency, fimp.If necessary data to calculate the impact frequency is lacking,the most critical frequency class given in Table 6-1 shall beselected.

For pipelines exposed to different types of trawl equipmentwith significant differences in mass and trawling frequency(e.g. trawling both with and without clump weights), qualifica-tion may be based on an acceptance criterion for each equip-ment type. Impact testing would then need to cover the totalnumber of test blows of all equipment categories. The numberof trawl equipment categories should be limited to three, toavoid unrealistic low frequency class per category.

6.2 Pipe shell - impactAn acceptable design against impact requires assuranceagainst the following failure modes:

— Denting: The out-of-roundness due to a dent should notprevent safe operation of internal inspection vehicles.

— Collapse: The capacity against collapse shall be checkedaccording to the collapse criterion in DNV-OS-F101 byusing an ovality which corresponds to the dent depth. Sucha failure mode will, however, mainly affect temporaryphases with external overpressure, e.g. installation or lossof pressure. May also apply for shut-downs, when the fluiddensity is less than for the external sea water.

— Fatigue: The stress concentration caused by a dent shall beaccounted for by a suitable SCF in fatigue calculations.

The maximum accepted ratio of permanent dent depth to theouter pipe steel diameter is:

where Hp,c is the characteristic permanent plastic dent depthand η is the usage factor.The acceptable permanent dent sizes are given in Table 6-2.No notch or sharp indentations is permitted. This shall beensured by the use of a coating which protects the steel pipefrom direct contact with any sharp edged part of the trawl gear.

The dent size shall be estimated by using the force-dent pipeshell relationship given in Section 4 or similar calculationsusing non-linear shell FE analysis as described in Appendix A.The dent size may also be measured during impact testing ofthe coated pipe.The effect of dents on additional failure modes for fatigue andcollapse shall be considered. Dents and especially dents withnotches in the girth weld area may have a significant effect onthe fatigue damage due to e.g. variations in the internal pres-sure.

6.3 Coating - impactCoating is used for protection of the steel pipe in addition topossible weight increase and thermal insulation.The protective coatings must be qualified by impact testingapplying the following:

— The energy absorbed by the coated pipe shell shall bequantified based on the impact response calculations inSection 3 taking into account the efficiency of the test rig.

— The number of test impact blows shall consider the trawl-ing frequency, see Table 6-3.

Note that the field joint coating must satisfy the same require-ments as for the general coating because the impact frequency

Table 6-1 Frequency classes for trawl gear crossingsFrequency class Impact frequency, fimp

[/year /km]High > 100

Medium 1-100Low < 1

a)

b)

c) (6.1)

Table 6-2 Acceptable dent sizes relative to outer diameterFrequency class Usage Dent depth, Hpc

[%] of DHigh (> 100) 0.0 0

Medium (1-100) 0.3 1.5Low (< 1) 0.7 3.5

Note:Acceptable dent depth also needs to comply with the requirements given in DNV-OS-F101.

Table 6-3 Number of test impact blows within 0.5 metre lengthFrequency class Impact frequency, fimp

[/year /km]Blows

High > 100 8Medium 1-100 4

Low < 1 1

η0.05/, =DH cp

DET NORSKE VERITAS


applies per unit length. The impact testing considers repeatedtesting within a single 0.5 m length. The probability of impactis equally distributed along each section of the pipeline. Hence,the probability of impact is equal for the 0.5 m long field jointsas for the pipe joints.The following functional requirements apply for the coatingincluding the field joint coating:

— the pipeline steel material shall be protected against dentslarger than acceptable

— the pipeline corrosion protection system shall remainintact, i.e. if damage to the corrosion coating exposing thesteel is not acceptable, this must be compensated by con-servative increase in cathodic protection

— acceptable weight loss shall be in accordance with therequirements for on-bottom stability of the pipeline.

Additional requirements valid for concrete coating:

— the inner reinforcement layer shall not be exposed— large areas (typically 200 x 200 mm) of concrete spalling

should be avoided. However, the acceptance criterion foracceptable spalling area is governed by cathodic protec-tion requirements for pipeline and coating steel reinforce-ment, and requirements related to resulting submergedweight of the pipeline. Provided these acceptance require-ments are met, larger spalling areas may be accepted.

Additional requirements valid for insulation coating:

— the insulation effect must not be degraded by trawl gearimpacts.

6.4 Pull-overThe trawl pull-over load may cause a lateral displacement andassociated bending of the pipeline. In combination with thecompressive axial force, trawl pull-over may lead to ratherlarge deflections and high utilisation of the pipeline steel mate-rial. Four scenarios are typically foreseen:

1) a pipeline with negligible effective axial force2) a pipeline with release of effective, compressive axial

force (postbuckling) prior to trawling3) a pipeline with release of effective, compressive axial force

simultaneously with trawling (i.e. the trawl load triggers theglobal buckling)

4) a free spanning pipeline.

Scenario 1 and 4 are covered within this RP. For scenarios 2and 3, more detailed assessments are required to establish theproper functional load condition factor including sensitivitystudies, analysing the pipeline response from pull-over loadsconsidering both upper- and lower bound lateral soil resist-ance, combined with the lower bound axial friction.Scenario 1 and 4 are covered within this RP, while scenarios 2and 3 are covered by this RP in combination with DNV-RP-F110.Figure 6-1 gives an overview of the safety factors to be appliedto the trawl pull-over load effect.

Figure 6-1Trawl pull-over load effect factors.

Guidance note:The annular trawling frequency threshold level where trawl pull-over loading may be disregarded is one order of magnitude belowthe acceptance criteria on total probability of failure, given inDNV-OS-F101. This is to account for that the considered pipe-line section may be exposed to additional accidental loads thantrawling.


As noted from Figure 6-1, the trawl pull-over load is combinedwith the design functional load (i.e. local design pressure andtemperature - i.e. not "maximum incidental pressure") for anannual trawling frequency per section larger than one.For pipeline sections with annual trawling frequency of lessthan one, the pull-over interference load shall be combinedwith the local normal operating functional loads (i.e. not "Max-imum Operating Loads", and based on appropriate tempera-ture- and pressure profiles along the pipeline, to reflect thelocal functional load per considered section).Further, for sections with annual trawling frequency of lessthan one, the trawl pull-over load may be reduced by a factorof 0.8 to account for the low probability that such low fre-quency trawling occurs with the combination of both the larg-est equipment, trawl direction perpendicular to the pipeline atmaximum trawling velocity and at the most critical position.See Table 6-4.

Note that the trawling frequency given in Figure 6-1 is notannual per kilometre as for the trawl impact frequency in Table6-1 but annual per relevant section. This frequency is calcu-lated according to Eq. (2.1) by considering section lengthinstead of per km.Section is here defined differently depending on pull-over sce-nario:

— pull-over response in the apex of a lateral buckle (post-buckling, scenario 2): The section length is taken as thelength of the sum of buckles if more than one buckle isanticipated. The length of the relevant section is typicallyless than 100 meters per buckle.

— pull-over response of not buckled pipeline (pre-buckling

Table 6-4 Trawl pull-over load factor for different trawl frequencies

fT >1 fT < 1Load factor 1.0 0.8Note: this is a load factor to be applied directly on the pull-over load calculated according to equations given in Sec. 4 above (i.e. and not on the load effect referred in Figure 6-1).

Traw

ling

Freq

uenc

y(A

nnua

l per

rele

vant

sec

tion)

10-6

10-5

10-4

10-3

10-2

10-1

1

101

Low Normal HighPressure andtemperature

Mtd= γA •γC•Mt = 1.0•γC•Mt

Disregarded

ALS

Mtd=γF •γC •Mt=1.1• γC•Mt

ULS

Des

ign

Ope

ratin

g

Safety Class

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scenario 1 and scenario 3): section taken as the length ofthe entire pipeline (order of kilometres) or a pipelinestretch where trawl gear crossing frequency, effectiveaxial force, soil resistance and other important parametersare more or less constant

— pull-over response in free-spans (scenario 4): sectionlength taken as the sum of the span lengths.

The condition load effect factor defined in DNV-OS-F101, γC,may be used to account for different reasons, e.g. due to:

— uneven seabed— pull-over with low probability of occurrence, and/or due to— sensitivity analysis for buckling pipelines.

In case several γC are relevant simultaneously, they should bemultiplied to account for all effects.For free spans on uneven seabed the γC applies for the verticalload component of the trawl load. This is to account for uncer-tainty in gap height, span length, soil support stiffness and pos-sible variations of these over time. The load condition factorfor the vertical load component of the trawl load shall be takenas γC = 1.07, unless sensitivity studies on relevant parameterscan justify a lower factor. The load condition factor for the hor-izontal component of the trawl load at free spans is γC = 1.0.The following requirements shall be fulfilled to ensure safeoperation of the pipeline during and after trawl pull-over:

— The trawl pull-over load effect shall be checked in combi-nation with other load effects (e.g. from temperature, pipe-line sagging, etc.).

— All relevant failure modes stated in DNV-OS-F101, i.e.local buckling, accumulated plastic strain, etc. shall bechecked. The usage factors for these failure modes aregiven in DNV-OS-F101 for each safety class.

— In free-spans, the span length and the gap may change fordifferent operating conditions. The trawl interference analy-ses should be performed for all relevant span configurations.

— A load effect factor equivalent to the functional load effectfactor (γF = 1.1) shall be used for the pull-over interfer-ence load with an annual trawling frequency >10-2. Foroccurrences less than once per hundred year, the pull-overinterference load may be considered as an accidental load(γA = 1.0). In case of extremely infrequent trawling, thepull-over load effect can be disregarded, see Figure 6-1.

— Possible accumulation of damage, i.e. strain, due to subse-quent trawling should be accounted for where applicable.The worst combination of subsequent trawling directionsshould be checked, i.e. all in the same direction or some inthe opposite direction.

The effect of concrete coating bending stiffness should be con-sidered, as localisation of bending may occur due to the dis-continuity of stiffness in the field joints.For pipelines exposed to different types of trawl equipment withsignificant differences in masses and trawling frequency (e.g.trawling both with and without clump weights), qualificationmay be performed for each of these - e.g. the pull-over load fromequipment with low trawl frequencies may be considered com-bined with appropriate operating loads (i.e. pressure and tem-perature), whereas equipment with high trawl frequency shouldbe combined with the design pressure and temperature.

6.5 HookingThe following requirements shall be fulfilled for hookingloads:

— The trawl hooking load effect shall be checked in combi-nation with other load effects where applicable (e.g. tem-perature effect loads, pipeline sagging, etc.).

— Due to the low probability of hooking, typical / normaloperational values can be used for pressure and tempera-ture.

— All relevant failure modes from DNV-OS-F101, i.e. localbuckling, accumulated plastic strain etc. shall be checked.The usage factors for these failure modes are given inDNV-OS-F101 for each safety class.

— Hooking shall be regarded as an accidental limit state, thusthe load effect factors for the hooking and other loadeffects shall be equal to unity, i.e. γΑ = 1.0.

— The hooking load effect shall be checked as a load control-led (LC) condition.

The effect of concrete coating should be considered, as thestiffening effect of the coating will lead to a longer lifted sec-tion, higher lifting forces and bending moments.For the lowest trawl gear impact frequency class, hooking isassumed to have a very low probability (i.e. annual hookingfrequency per pipeline of less than 10-4 to 10-5 per year) andcan be neglected.

7. Coating Impact Testing7.1 GeneralThe following describes a method for qualification of the pro-tective coating with respect to trawl impact. The methodimplies testing of coated pipe sections and covers the follow-ing items:

— specimens to be tested— test equipment— test procedures— calibration of test equipment.

7.2 SpecimensTypes: Coated pipes with relevant materials and dimensions.

The coating shall be documented by specifying the coatingtype, possible reinforcement and the application procedure.For quality control of the coating during production or applica-tion, samples should be tested when new batches of coating areapplied. New batch is in this context defined as a batch forwhich a possible change in the production procedure canchange the final mechanical properties.Concrete coating shall have achieved its twenty-eight daystrength before testing.In case the coating is sensitive to temperature, the impact testshall be carried out both at maximum and minimum tempera-tures.The effect of ageing during the design lifetime should beaccounted for. The field joint coating must satisfy the same requirements asfor the general coating. Alternatively, documentation fromprevious tests on similar pipes may qualify the field joint coat-ing material.For pipelines with attached heating cable protection cover, thiscover could be qualified by impact testing separately from thepipeline with coatings. The protection cover test should thenbe performed on rigid foundation, and include the heatingcable. The test energy level needs to be derived from represent-ative analyses, reflecting the tested configuration as illustratedby Figure 7-1.

Number of specimens per coating type: minimum 3Length of specimens: minimum 2xOD

DET NORSKE VERITAS


Figure 7-1Typical load versus displacement plot for a pipeline with coatingand attached DEH cable with protection structure

The following two impact test requirements apply for the DEHcable with protection structure attached to the considered pipesection:

1) Impact test energy level ETest = EDEH + EPipe. Alterna-tively, if the DEH cable with protection structure is testedseparately from the pipe on a rigid foundation, the testenergy may be limited to ETest = EDEH.

2) Minimum required impact force induced by the test ham-mer shall be FTest (see Figure 7-1), to document that thesystem including cable sustains this load level withoutunacceptable damage.Alternatively, the load capacity can be documented by astatic load versus deformation test.The value of the FTest needs to be established based on rep-resentative analyses, as described in Appendix A.

The load versus deformation curve may vary for different pro-tection structure designs, and needs to be established by testsfor each type.

7.3 Test equipment

7.3.1 GeneralA typical test rig is shown in Figure 7-2.

Figure 7-2Typical test rig outline

The kinetic energy of the hammer is:

where mh is the mass of the hammer, g is the gravitationalacceleration and Hi is the pendulum height.

7.3.2 HammerThe mass of the hammer shall reflect the mass of the trawlequipment that the test is to represent.The hammer shall be designed to accommodate the impactforces without significant flexing or permanent deformation.The front of the hammer should ideally have a shape represent-ative of the trawl equipment. Normally both trawl boards andbeam trawls (with hoop bars) have rounded frontal shapes.Further, the structural frame of the largest clump weights ismade from 40 - 50 mm plates.A rectangular plate of 300 mm height and 50 mm width with aflat or half-round front may generally be used, see Figure 7-3.

EDEH EPipe

FTest

Force

Deformation

(7.1)

P

m1

m2

Hammer

ihkin gHmE =

DET NORSKE VERITAS


Figure 7-3Hammer front shapes

However, due to damage and repair of the trawl gear, sharpedges may occur and should be considered. A rectangular plateof 300 mm height and 50 mm width with a conical shape andan edge radius of 10 mm may be used.

7.3.3 Pipe supportsThe pipe support system should be adequately stiff and mas-sive so that the energy is absorbed by the pipe and coating andnot by the rig.The mass of the supports should exceed 10 times the mass ofthe hammer.The stiffness of the supports should exceed 10 times the initialshell stiffness of the pipe and/or the initial coating stiffness ofthe specimens to be tested. Practical solutions to obtain this stiffness are:

— to mount a stiff mass behind the pipe in the impact direc-tion (see Figure 7-2, mass m1)

— to attach the rig to a massive floor (see Figure 7-2, massm2). The attachment to the floor must be designed toaccommodate the maximum impact forces imposed by thehammer.

Brackets "B" between the pipe and the rig supports shall be"stiff" (steel) and give an even support of the pipe. The brack-ets shall extend over a length of at least 0.5D and a circumfer-ence of at least 30o. There shall be means to pre-tighten thespecimens to the brackets and the rig.The requirements for the pipe support as given above are validfor impact energies calculated according to the simplifiedmethod as given in Section 3 and the advanced method asdescribed in Appendix A. However, the impact energy to be applied in the tests may becalculated without the reduction factors as given in Section 3.

Then the pipe supports are allowed to be more flexible, e.g. asimple support span of 1-2 metres supported on woodenbeams.

7.4 Test procedureDetermine:

— the test impact energy based on the qualification impactenergy and the test rig efficiency factor

— the numbers of impacts required for each specimen, seeTable 6-3.

Determination of impact resistance by repeated impact tests:

— repeat the testing until unacceptable damage according toSection 6.2 and 6.3 is detected. Each impact is performedat a location on the specimen as defined in Figure 7-4.

Figure 7-4Positions for repeated impacts

— test several specimens and determine the impact capacityresistance by one of the following methods:

1) At least 3 impact tests on different pipes are per-formed. The capacity is set equal to the lowest numberof impacts before unacceptable damage:

2) At least 6 impact tests on different pipes are per-formed. The mean and standard deviation for thesetests are calculated and the characteristic impactcapacity determined as:

Here ncap is the capacity of the coated pipe in terms of numberof impacts before failing, nmean is the mean number deter-mined in testing while σ is the standard deviation.

7.5 Test-rig calibrationSome of the kinetic energy will be absorbed by the flexibilityof the rig and the pipe support system as the rig is not 100%efficient. Therefore, the impact energy applied in the tests shalltake into account the efficiency of the rig.The efficiency depends on the rig design and the design of thebrackets or interference with the coating or pipe shell. The effi-ciency of the test rig may be calculated based on the mass andstiffness properties of the rig or estimated from dynamic meas-urements. Hard concrete coated specimens, supported by solid steel, pre-tightened brackets as in Section 7.3 is considered to be practi-cally 100 % efficient.The efficiency of polymer coated specimens may be deter-mined by aid of static or dynamic measurements. Alternativelythe coating may be removed and the pipe supported as sug-gested in Section 7.3.

(7.2)

(7.3)

500 mm

3,2,1)min( == inn icap

σ−= meancap nn

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8. Bibliography

[1] Fyriliev, O., Spiten, J., Mellem, T. and Verley, R., “DNV’96, Acceptance Criteria for Interaction between Trawl Gear and Pipelines”, OMAE, pp. 91-98, Yokohama, Japan, 1997.

[2] Guijt, J. and Horenberg, J.A.G, “Recent investigations con-cerning the effect of bottom trawl gear crossings on subma-rine pipeline integrity”, Offshore Tech. Conf., Paper OTC 5616, 1987.

[3] Kjeldsen, S.P. and Moshagen, H. “Influence of Bottom Trawl Gear on Submarine Pipelines. Extension III. A Sum-mary of Laboratory and Field Investigations”, VHL rapport, STF60 F79012, 1979.

[4] Mellem, T., Spiten, J., Verley , R. and Moshagen, H., “Trawl Board Impact on Pipelines”, OMAE, pp. 165-178, Firenze, Italy, 1996.

[5] Moshagen, H. and Kjeldsen, S.P., “Fishing Gear Loads and Effects on Submarine Pipelines”, OTC 3782, Offshore Technology Conference, Houston, 1980.

[6] Nygaard, I., “Trawl - Pipeline Span Interaction. Model Tests. Final Report”, Marintek report no. 511191.06, 1990.

[7] Verley, R.L.P. et al., "Trawl forces on Free-spanning pipe-lines", ISOPE, 2, No.1, 1992.

[8] Verley, R.L.P., "Pipeline on a flat seabed subjected to trawl-ing or other limited duration point loads", Proc. Int. Offshore and Polar Eng. Conf., 1994.

[9] Fyrileiv, O., Spiten, J. “Trawl gear protection within plat-form safety Zones”, paper OMAE2004-51406.

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APPENDIX A ADVANCED IMPACT CALCULATION METHOD

A.1 IntroductionPhysical effects that absorb kinetic energy during the trawlgear impact are:

— local deformation of the pipe steel wall— local deformation of the coating— local deformation of other elements that absorbs trawl

impact energy (e.g. piggy-back heating cables inside pro-tection structure)

— global pipe bending— pipe inertia effects inclusive hydrodynamic added mass— trawl board, beam or clump weight deformation— friction between pipeline and soil— soil deflections.

These effects may be included in an analysis model and adynamic simulation performed to reduce the uncertainty andconservatism in the simplified approach.Further, other pipeline configurations not covered by the sim-plified method (ref. Section 3) may be covered by analyses:

— flexibles— pipelines with other coatings (e.g. polypropylene, polyeth-

ylene, field joint coatings)— piggyback electrical cable strapped to the pipeline— electrical cable protection structure

A.2 Analysis ModelTrawl boardsFor trawl boards the effect of impact is split into two parts; oneassociated with the in-plane velocity of the steel mass of theboard, and a second, which will generally occur some hun-dredths of a second later, caused by the hydrodynamic addedmass acting through the flexural stiffness of the board.Beam trawlsFor beam trawls the in-plane deformation and stiffness of thebeam is of importance. However, as the beam is spanning 10-20 metres, the effect of the mass of the beam and the shoe inthe opposite end will gradually increase with time. Therefore,to represent the bending of the beam correctly it is recom-mended that the beam is modelled with beam elements andconcentrated masses at each end to represent the beam shoes.Clump weightsThe clump weights used differ in shape and mass. However,the largest clump weights used in the North Sea and in the Nor-wegian Sea are the roller type. Two typical impact scenariosare illustrated in Figure 3-2. The following effects should beconsidered in an advance impact analyses with clump weights:

— Dynamic simulation, to calculate the input impact energywhen the clump weight hits at one corner, to account forthe induced rotation of the clump weight.

— Local stiffness of clump-weight, considering both impactat corner and impact at mid-span of clump weight. (Simpleanalyses give a stiffness of approximately 4200 MN/m forthe corner plate, and 22 MN/m for a typical front framebeam before interfering with the drum, exposed to a singlepoint load at mid-span).

Added mass of one clump weight can be calculated accordingto the methodology given in DNV-RP-C205. Typical mass ofentrained water in clump weights are given in Table 3-1.Analyses model assemblyThe principles of the analysis model are shown in Figure A-1for trawl boards. A similar model is applied for beam trawls

and clump weights except that the lateral stiffness and the asso-ciated added mass are omitted. In addition the kinematics ofbeam/clump will cause a time dependent effective mass andstiffness that may be accounted for in an analysis.Here:— ma and mt are the hydrodynamic added mass and steel

mass of the trawl gear— kb and ki the out-of-plane and in-plane stiffness of the gear— kc1 represent the stiffness of the protective cover for heat-

ing cable attached to the pipeline, when applicable— kc2 is the stiffness of coating— kc3 represents any possible effect it has on the steel shell

stiffness by distributing the impact force over a larger areaby shear deformations in the coating

— ks is the local shell stiffness of the steel pipeline— mp is the effective mass of the pipe, including hydrody-

namic added mass effects— kpb is the effective bending stiffness of the pipe— kps is the effective soil stiffness acting on the pipe.The effective masses and effective velocity to be applied in thedynamic analysis may be obtained from simulations of thetrawl impact or from the simplified, conservative values givenin Table 3-1. The pipe mass, bending stiffness and soil resistance involvedin the interaction all vary with time as more of the pipe adja-cent to the interaction point is involved. This pipe and supportsub-system may most conveniently be modelled as beam ele-ments, see Figure A-2. Note that the effective masses and impact velocity are based onan impact direction perpendicular to the pipe. Due to symme-try the problem may be reduced to a half model.A sufficient length of the pipeline must be included in thestructural model to avoid end effects influencing the results. Alength of the half model in the order of L/D = 50 is found to besufficient for a 16” pipeline. However, as the necessary lengthmay vary with other factors as weight etc., it should be verifiedthat the model has a sufficient length.

A.3 Trawl Board StiffnessIt should be noted that the shell stiffness of a pipeline wouldnormally be in between the trawl board in-plane and bendingstiffness. This implies that the impact energy associated withthe in-plane steel mass will mainly be transferred to the pipe,while the impact energy associated with the added mass willpartly be absorbed by flexing of the trawl board. The amountabsorbed by the bending action of the trawl board depends onthe relative stiffness between the pipe shell and the trawlboard.The trawl board stiffness kb and ki can be estimated through FEanalyses, however the values given in Table 3-1 may be usedfor all trawl boards if no other information is available.

A.4 Pipe Shell StiffnessIf the pipe shell is not modelled directly, the coating and shellstiffness may be obtained from tests or separate analysis.These are applied as non-linear springs attached to the pipebeam model. The shell stiffness must also account for the front shape of thetrawl board or shoe, as the geometry of the contact area willsignificantly influence the relation between the contact forceand the pipe indentation.If no detailed information is available, the local stiffness of aconcrete coated pipe may conservatively be approximated bythe following relationship between the impact force and the

DET NORSKE VERITAS


indentation depth of a bare steel pipe:

Here, Fsh is the impact force experienced by the pipe shell, t isthe wall thickness and Ht is the dent depth. (i.e. both elastic andplastic).The basic assumption for the above relationship between thecontact force and the indentation is that the concrete coatedpipe is stiffer than the steel shell. Thus, the estimated stiffnessis lower than the real one and the calculated absorbed energybecomes larger. In this way the effect of the coating is conserv-atively neglected. For soft coatings this relationship may leadto a non-conservative result as the energy absorption becomeslarger than for the bare steel pipe.The energy absorbed locally by the steel pipe can be calculateddirectly from the maximum force obtained from the impactanalysis as:

A.5 Soil StiffnessThe soil stiffness, kps, is the effective stiffness which is to bedetermined dependent on the soil type. Analyses should be

based on conservative values of the upper- and lower boundsoil stiffness (i.e. with respect to pipeline response).

A.6 MassesIf the pipe shell is modelled by shell elements, the effect of thecontent and the hydrodynamic added mass can be accountedfor as a smeared mass over the shell.When calculating the hydrodynamic added mass of the pipe-line, the span height over the seabed shall be accounted for byappropriate methods as e.g. in DNV-RP-F105.

A.7 DEH cable with protection structureThe stiffness of the DEH cable protection structure, with theelectric cable located inside, may be obtained from tests, pro-viding non-linear effects such as when the protection structureis fully deformed and in contact with the electric cable.The stiffness of the DEH cable with protection structure maybe affected by different combinations of impact velocities,mass, and water filling of the protection structure. The test pro-gramme to establish this stiffness should reflect these parame-ters. For systems where the stiffness is sensitive to the impactvelocity, the applied test impact velocity needs to comply withthe established trawl velocity of the considered area.The non-linear spring representing the DEH cable with protec-tion structure is included in the analysis model at the impactlocation, as shown by spring stiffness kc1, in Figure A-1.

Figure A-1 Principles of the analysis model for trawl gear impacting a pipeline. Note, the hydrodynamic added mass for trawl boards is appliedthrough the out-of-plane stiffness, whereas for clump-weights the added mass is added directly to the mass of the clump weight (i.e. forclump weights, the total mass equal (ma + mt), and associated stiffness equal ki).

A.1

A.2

21

23

5 tysh HtfF ⋅⋅⋅=

32

3

75

2)(

tf

FdHHFE

y

shttshab

⋅⋅

⋅=∫=

ma

mt

kb

ki

kc1 kc2 ks

kc3

mp

kps

kpb

Trawl equipment Coating and shell Pipe & Support

DET NORSKE VERITAS


Figure A-2 Typical scheme for simulation of the impact process

DentPerform static analysis. Dentingforce includes effect of trawlequipment geometry at impact point,and possibly DEH cable protectionstructure and/or possible coating.

2

4

DEH cable with prot. structurestiffness

Local pipe shellstiffness

1

3

Force

Time

Force

Dent

Perform dynamic analysis. Pipeshell performance included asnon-linear spring. Trawlequipment stiffness included assprings. Pipe global bendingstiffness included by use ofbeam elements. Soil stiffnessmodelled with discrete groundsprings both vertical and lateral.

Establish force-dentrelationship of thelocal pipe shell.

Trawl equipmentstiffness and mass bothin- and out-of-plane /bending, given initialimpact velocity anddirection.

Force

Calculate impact energyabsorbed locally by the pipeshell, coating and DEHprotection structure based onthe maximum impactingforce and the establishedforce-dent relationship.

DET NORSKE VERITAS


APPENDIX B EXAMPLE

Design basisAs an example a 13 km 14” pipeline is chosen to demonstrate the use of the RP.

Trawl equipment and fishing intensity see (Section 2)The pipeline is operated in an area exposed to consume trawling where the largest (4000kg) polyvalent board is used. No beam trawling is expected in the area. The fishing vessel density (i.e. number of trawlers per area) for the field is found by a separate study to be 0.4 per 103 km2 as a mean for the whole pipeline route. A predominant trawling direction of 70° to the pipeline is found due to the bottom topography. The whole pipeline length is exposed to trawl loads.

Pipeline dimensions and material (general) Pipeline dimensions Outer diameter, D ............................................................ : 356 mm Wall thickness, tnom .......................................................... : 16 mm Corrosion allowance, tcorr................................................. : 3 mm Steel material Steel quality ..................................................................... : SML450I U Specified minimum yield stress....................................... : 450 N/mm2 Specified minimum tensile strength................................. : 535 N/mm2 Derating at design temperature ........................................ : 0 MPa Coating Type................................................................................. : Concrete Thickness ......................................................................... : 40 mm Specific weight ................................................................ : 1900 kg/m3 Content Content ............................................................................ : OIL Specific weight ................................................................ : 800 kg/m3 Design temperature.......................................................... : 40 ºC Design pressure................................................................ : 100 barg Environmental data Water depth, d.................................................................. : 300 m Ambient temperature ....................................................... : 5 ºC Soil conditions Sand, friction angle, φ ..................................................... : 35 deg. Axial friction coefficient.................................................. : 0.4 Lateral friction coefficient ............................................... : 0.6 Safety Philosophy Fluid category .................................................................. : B Location class .................................................................. : 1 Safety class ...................................................................... : Normal

Trawl board type ......................................................................... : Polyvalent Trawl board steel mass, mt .......................................................... : 4 000 kg Trawl size (length × height) ........................................................ : 4.5 × 3.5 m Trawl velocity, V......................................................................... : 2.8 m/s Warp line length, Lw.................................................................... : 3 × d = 900 m Trawling frequency (see section 2.4) Vessel density, I .......................................................................... : 0.4 per 103 km2 The trawl gear impact frequency is calculated as (see Equation 2.1):

kmyearperkmhrperhrkm

kmVInf o

egimp −=−=⋅⋅=⋅⋅⋅⋅= 640076.0)20cos(1.1010

4.02cos23

ϕα

DET NORSKE VERITAS


Impact (see Section 3)The simplified approach is used to obtain a conservative result. The pipeline is coated with normal density concrete that is stiff enough to conservatively assume a shell stiffness of the coated pipe at least equal to that of a bare steel pipe. The pipe is resting on the seabed with no free spans.

Figure B-1: Impact force versus time, obtained from dynamic finite element analysis

Pipe wall thickness: t = tnom – tcorr = 16 – 3 = 13 mm

Absorbed impact energy (see section 3.3) The absorbed energy due to the impacting steel mass is calculated as (see Equation 3.1):

kJVCmRE htfss 2.6)8.285.0(400055.0)( 2212

21 =⋅⋅⋅=⋅=

The impacting force caused by the hydrodynamic mass is calculated as (see Equation 3.2):

kNkmVCF bahb 700)10108600(8.285.0)( 5.065.0 =⋅⋅⋅⋅==

which then gives the absorbed energy of the hydrodynamic mass (limited by the maximum energy available, see Equation 3.3):

221

32

3)(

75)(2 VCm

tfFRE hay

bfaa ⋅≤

⋅⋅⋅=

221

3326

33)8.285.0(8600

)1013()10450(75

)10700(215.0 ⋅⋅≤

⋅⋅⋅⋅

⋅⋅⋅=

−aE

kJkJEa 4.241.3 ≤=

The absorbed energy is the maximum of Es and Ea, i.e. 6.2 kJ. This estimate is conservative, especially for smaller pipelines; hence an analysis according to the model given in Appendix A is performed.

Dynamic analysis:

Impact - Force vs. time

Max. 395 kN

0

50

100

150

200

250

300

350

400

450

0 0.02 0.04 0.06 0.08 0.1Time [s]

Impa

ct fo

rce

[kN

]

Trawl equipment, additional data for impact calculations (see Table 3-1) Trawl board type ......................................................................... : Polyvalent Trawl board hydrodynamic mass, ma=2.14•mt ........................... : 8 560 kg Trawl board bending stiffness, kb................................................ : 10 MN/m Trawl impact velocity coefficient, Ch ......................................... : 0.85 Energy reduction factors (see Figure 3-3) Steel mass associated, Rfs............................................................ : 0.55 Added mass associated, Rfa ......................................................... : 0.25

DET NORSKE VERITAS


Pull-over (see Section 4)The pull-over scenario is analysed using a dynamic finite element analysis. To focus on the pipeline response from the pull-over load only in this example, the pipeline is considered to have released the residual forces by global buckling close to the pull-over location. Hence, the considered pipeline has negligible compressive forces due to thermal and internal pressure effects, and the analyses are based on zero operational pressure and ambient temperature in this example.

Impact force, obtained from FE analysis, see Figure B-1: .......... : 395 kN Energy absorption, Equation A.2 ................................................ : 3.7 kJ Permanent dent depth, Hp,c.......................................................... : 9.0 mm Permanent dent depth, as % of outer steel pipe diameter............ : 2.5 %

The impact energy to be applied in the testing of coated pipes is reduced from 6.2 kJ to 3.7 kJ using the advanced method given in Appendix A. Note: The dent depth is calculated with a pipe shell stiffness as for a bare steel pipe, i.e. neglecting the stiffness contribution from the concrete coating. Therefore, this value which slightly exceeds the acceptance criteria must be considered as a conservative estimate. It is believed that the dent depth found during impact testing of the coated pipe will be well within the acceptable limits according to section 6.2.

Pull-over load (see section 4.3) Dimensionless height (see Equation 4.5):

239.075.1

2.02/436.002.02/=

++=

++=

B

ODHH sp

The empirical force coefficient (see Equation 4.3):

39.1)1(0.8)1(0.8 239.08.08.0 =−⋅=−⋅= ⋅−− eeC HF

The warp line stiffness is estimated as (see Equation 4.8):

mkNL

kw

w /393003105.3105.3 77

=⋅⋅

=⋅

=

The maximum pull-over force becomes (see Equation 4.2):

kNkmVCF wtFp 6.48)10394000(8.239.1)( 2/132/1 =⋅⋅⋅⋅=⋅=

The corresponding maximum downward acting force becomes (see Equation 4.9):

kNeeFF Hpz 1.31)8.02.0(6.48)8.02.0( 239.05.2)5.2( =⋅+⋅=⋅+= ⋅−⋅−

Pull-over load duration (see section 4.5) Pull-over duration (see Equation 4.19):

sVkmCCT pwtFTp 98.0)39000/4000(39.10.2)1.01(/)/( 2/12/1 =⋅⋅⋅+=+= δ

Trawl equipment, additional data for pull-over calculations: Water depth, d............................................................................. : 300 m Trawl board height, (h = 2B) ..................................................... : 3.5 m Span height, Hsp .......................................................................... : 0.0 m Warp line diameter...................................................................... : 38 mm Drag coefficient .......................................................................... : 2.0 Added mass coefficient............................................................... : 2.0 Axial friction coefficient ............................................................. : 0.4 Lateral friction coefficient .......................................................... : 0.6 Load effect factor ........................................................................ : 1.1 Condition load effect factor, γC ................................................... : 1.07 Note: the load effect condition factor, γC, is to be applied on the vertical component of the pipeline response from the trawl pull-over load, i.e. to the moment acting about the horizontal axis perpendicular to the pipeline axis. The functional load factor, γF, is to be applied to the total pipeline response from the pull-over load, i.e. both the axial force and moment response from the pull-over load.

DET NORSKE VERITAS


Acceptance criteria (see Section 6.4): The resulting pipeline effective axial force before the pull-over load is applied, is -81 kN. The loads specified in the following reflect the total response from the finite element analysis:

Figure B-2: Pipeline response, obtained from dynamic finite element analysis

1st.

Displacement

-0.050

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 5 10 15 20 25

Time [sec]

Disp

lace

men

t [m

]

- Z - Y

2nd. 3rd. 4th.Pull-over load

1st.

Displacement

-0.050

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 5 10 15 20 25

Time [sec]

Disp

lace

men

t [m

]

- Z - Y


Displacement

-0.050

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0 5 10 15 20 25

Time [sec]

Disp

lace

men

t [m

]

- Z - Y


Moment - Trawling

-2.00E+05

-1.50E+05

-1.00E+05

-5.00E+04

0.00E+00

5.00E+04

1.00E+05

1.50E+05

2.00E+05

0 5 10 15 20 25

Time [sec]

Mom

ent [

Nm

]

- MXY - MXZ - Msqrt

A B

First pull-over (dynamic analysis): Effective axial force (functional) ................................................ : -69 kN Maximum bending moment (functional) .................................... : 113 kNm Maximum stress .......................................................................... : 94 MPa Utilisation, local buckling (at time “A” in fig. B-2, ref. OS-F101) ....... : 0.38 (OK) Fourth pull-over (dynamic analysis): Effective axial force (functional) ................................................ : -40 kN Maximum bending moment (functional) .................................... : 148 kNm Maximum stress .......................................................................... : 124 MPa Utilisation, local buckling (at time “B” in fig. B-2, ref. OS-F101) : 0.46 (OK) Note: Four subsequent pull-overs at the same location and in the same direction are considered to give a conservative estimate for the effect of subsequent pull-overs for this pipeline that rests at seabed. The lateral displacement increases for each pull-over. However, the pipeline material still behaves linear elastically.

DET NORSKE VERITAS


Hooking (see Section 5)The pipeline has no free span, therefore only part penetration of a board has to be considered.

Impact testing of coated pipe (see Section 7)

Trawl equipment, additional data for hooking calculations: Load factor .................................................................................. : 1.0 Lifting height (see section 5.5): Maximum lifting height (see Equation 5.4): mODBH l 09.1436.03.075.17.03.07.0 =⋅−⋅=⋅−= Result from static finite element analyses, lifting height H l = 1.09 m: Lifting force ................................................................................ : 73 kN Effective axial force (functional) ................................................ : 784 kN Maximum bending moment ........................................................ : 340 kNm Maximum stress .......................................................................... : 290 MPa Acceptance criteria (see section 6.5): Load effect factor ........................................................................ : 1.0 Condition load effect factor, γC .................................................. : 1.0 Load condition ............................................................................ : Load controlled Corrosion allowance subtracted from nominal wall thickness.... : No Utilisation, local buckling (ref. OS-F101) .................................. : 0.53 (OK)

Test equipment, additional data for impact testing: Test rig efficiency (assumed)...................................................... : 0.7 Test input (see section 7.3) Impact energy, (i.e. Impact energy / Test rig efficiency) ............ : 5.3 kJ Hammer weight........................................................................... : 4 000 kg Hammer velocity......................................................................... : 1.69 m/s Hammer height ........................................................................... : 0.135 m Acceptance criteria (see section 6.2 or 6.3) Minimum number of impacts...................................................... : 4 Dent depth (steel), as % of outer steel pipe diameter:................. : 1.5 % No exposure of the inner reinforcement layer. No large concrete area spalling off (maximum 200 × 200 mm).

DET NORSKE VERITAS


DET NORSKE VERITAS

DNV-RP-F111: Interference Between Trawl Gear and...

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